Oh, wait. I just found another interesting item. Some reporter actually asked a otolaryngologist (ear, nose, and throat specialist), as well as an expert on manga for a
semi-serious little article on "R25.jp" ("The web oasis that sets men free" :snicker

Dr. OHKUBO Kimihiro of the Department of Otolaryngology at the Nippon Medical School Hospital said there's no medical connection. However, "when one is excited, blood tends to rush to the face and head, so it isn't entirely inconceivably, but could only happen in someone with high blood pressure who is prone to nosebleeds."
So then the reporter asked manga writer OHNISHI Shohei where this came from. According to Ohnishi, it started with the popular 1970s gag manga artist TANIOKA Yasuji, whose manga "Mettameta gakidou kouza" popularized the phrase "Hanaji buuuu!!" ("Hanaji" means "nosebleed" and "buuu!!" is the sound of the blood spurting out.) But in Tanioka's manga, it wasn't just sexual arousal, but anger or just being punched that would result in "Hanaji buuuu!!" :nosebleed: Ohnishi says this was picked up by manga artists in the 1980s, such as "Dr. Slump" and "Dragonball" creator TORIYAMA Akira and "Kickoff" creator CHIBA Taku, who used it exclusively to represent sexual arousal.
I'm actually a bit disappointed. I was hoping to find references dating from the Edo Period or even earlier, but it seems to be less than 40 years old. :dunno: